Audio cables for interconnecting equipment, commonly referred to as interconnects, typically carry signals of 1 volt or less, including signals as low as 0.25 millivolts. These low-level signals can be easily distorted by capacitive, inductive, and dielectric effects. Additionally, as audio signals typically cover a wide frequency range of 10 octaves from 20 Hz to 20 kHz, propagation velocity of a signal through the interconnect may vary widely, depending on dielectric material. Specifically, the characteristic impedance of a cable Z0 is defined as:Z0=[(R+j2πfL)/(G+j2πfC)]1/2 with resistance R, conductance G, inductance L, capacitance C, imaginary unit j, and frequency f. Within the typical human audible range of around 20 Hz to 20 kHz, R is typically much larger than j2πfL and j2πfC is typically much) larger than G, so the cable impedance can be simplified as:Z0=[R/j2πfC]1/2 Accordingly, cable impedance at 20 Hz may be drastically different than impedance at 20 kHz, three orders of magnitude higher.
Dielectric material around a conductor will affect the propagation velocity of signals in the conductor. Specifically, the velocity factor VF or ratio of the velocity of the signal in the conductor to the velocity of a signal in vacuum (i.e. the speed of light, c) is the reciprocal of the square root of the dielectric constant of the material (e.g. 1 for vacuum). Air has a dielectric constant only slightly above that of vacuum (e.g. roughly 1.00059 at standard temperature and pressure). However, conductors surrounded or separated by air may be impractical: such conductors may need to be rigidly fixed in place to avoid short circuits or variations in geometry or spacing, leading to changes in capacitance. Accordingly, many cables employ polyethylene or similar material for structural support. For example, many coaxial cables surround a center conductor with a polyethylene foam, supporting an outer conductor. By using a foam containing a large portion of air, the dielectric constant of the material is reduced compared to solid polyethylene. However, the velocity factor of such cables may still be approximately 80%. As with self inductance or impedance effects, propagation velocity is similarly frequency dependent and, with wide differences between arrival times of low frequency components and high frequency components of an audio signal, can result in audible phase distortion and “smearing”.